Field of the Invention
The present invention relates to C-arm X-ray imaging systems, and in particular, relates to methods for recording scans in a region of interest for CT reconstruction using C-arm X-ray imaging systems.
Description of the Related Art
In interventional X-ray diagnostics, mobile C-arm X-ray apparatuses are being used to an increasing extent. Such a C-arm X-ray apparatus is movable on the floor and it carries, on a chassis, a multiply adjustable holder in which a circular arc-shaped C-arm can be adjusted along its periphery in an orbital movement, wherein the C-arm carries an X-ray source at one end and an imaging X-ray detector at the other end, preferably a flat panel detector (FPD). It is preferable for all the adjustment axes to be provided with electrically controllable drives, so that, by means of a movement control, the X-ray recording unit consisting of the X-ray source and the X-ray detector can be positioned in the room and/or moved along a focus trajectory. After a positioning of the X-ray recording unit or also during the movement on a focus trajectory, X-ray projection views are prepared. If the X-ray detector has a round inlet window, such as, for example, in the case of an X-ray image amplifier or in the case of a round FPD, then the radiation field between the focal spot of the X-ray tubes is conical; in the case of the use of a rectangular FPD, the radiation field is pyramidal. In both cases, the term cone beam geometry (“cone beam,” abbreviated “CB”) is used in the literature. The radiation field is collimated in such a manner by a primary radiation diaphragm that all the rays of the radiation field fall onto the inlet window of the X-ray detector. If an examination object is introduced into the radiation field, then an X-ray projection of the spatial region of the examination object located within the radiation field can be recorded. In order to delimit the radiation field to a region of interest (Region of Interest, ROI), a diaphragm system that can preferably be adjusted by means of a motor is arranged between the focal spot and the examination object. All the image recording processes are controlled by an image recording control that is synchronized with the movement control. The recorded X-ray projection views are processed together with the data from the movement control and the image recording control in an image processing computer.
In the case of interventional X-ray diagnostics, the region around the patient bench is occupied by a number of apparatuses and, in addition, a working area must be provided for the persons performing the intervention or assisting during the intervention. A mobile C-arm X-ray apparatus used for the interventional diagnostics is moved toward the examination object, preferably approximately perpendicularly to the longitudinal axis of the patient bench, so that the C-arm plane contains the ROI approximately. In this work position, the wheels of the chassis are preferably blocked and the X-ray recording unit of the C-arm X-ray apparatus is moved by means of several adjustment axes, preferably controlled by means of a motor, into the desired position and alignment. If scans for recording an image series of projection images are produced during the intervention, then it is desirable that the movement of the X-ray recording unit and of the central ray remains in an initially set plane. Each movement component perpendicular to the original C-arm plane would increase the space requirement of the C-arm X-ray apparatus in the direction of the longitudinal axis of the patient bench and generate an increased risk of collision with other apparatuses and/or reduce the working space for the persons participating in the intervention. For handling the C-arm X-ray apparatus, it is particularly advantageous if the C-arm plane is vertical in the room. Then, only a small corridor needs to be kept clear as movement space for the movement of the X-ray recording unit during a scan. When the mobile C-arm X-ray apparatus is not in use for a brief time, it can be moved on wheels attached to a chassis on the floor from the patient bench approximately perpendicularly to the longitudinal axis of the patient bench from the latter into a parked position, and from said parked position it can be quickly moved back into the work position.
For the reconstruction of the X-ray volume of an ROI, the image data of a series of 2D X-ray projections of the ROI are needed, which have been recorded with different X-ray projection geometries. Here, the X-ray source and the imaging X-ray detector, for example, a flat panel detector FPD, move around the ROI, wherein, during the movement, X-ray projection views of the examination object are prepared.
With mobile C-arm X-ray apparatuses, it is preferable to record short scans if the rotation angle range is smaller than 360°. If the C-arm X-ray apparatus has an isocentric C-arm, in the case of which the central ray extends through the circle center of the C-arm, it is possible, by rotating the C-arm around its center, to record a flat rotation scan whose rotation angle range depends on the arc length and the radius of the C-arm.
If the C-arm X-ray apparatus has a non-isocentric C-arm, in which the circle center of the C-arm is located within the segment of a circle formed by the central ray and the C-arm profile, then, with such a non-isocentric C-arm X-ray apparatus, a rotation scan can be recorded as with an isocentric C-arm, if the X-ray recording unit is moved around a virtual isocenter in such a manner that the holder of the C-arm is adjusted in the C-arm plane for each scanning position in such a manner that the central ray extends through the virtual scan center. Here, the adjustment of the C-arm holder can occur in such a manner that the distance from the inlet window of the X-ray detector to the virtual scan center is the same for every projection geometry. However, it is also possible to record scans with variable distance between the inlet window of the X-ray detector and the virtual scan center with equal distance between the X-ray tube assembly and the X-ray receiver.
In order to be able to determine the 2D model of the X-ray absorption of the voxels of a disk-shaped ROI having the thickness of one voxel, from a set of X-ray projections using analytical computation methods, a complete projection data set is required. In this projection data set, for each voxel of the ROI, the integrals of the X-ray absorption values are available for all the projection lines in an angle range from 0° to 180°.
A complete projection data set is obtained for a disk-shaped ROI in the C-arm plane if the ROI is acquired completely by a fan beam and if the X-ray recording unit associated with the fan beam is rotated around the center of the ROI with a rotation angle range of 180° plus fan angle.
Complete projection data sets for the reconstruction of a 3D ROI can only be recorded with non-flat trajectories of the focus of the X-ray source. Thus, for example, short spiral scans, circle+line scans or circle-arc scans can be recorded with a C-arm X-ray apparatus, if the adjustment of the C-arm holder allows a movement perpendicular to the C-arm plane.
A projection data set that is complete for a disk-shaped ROI in the central layer and that was recorded with a cone beam geometry can also be used outside of the central layer as an approximation by applying a Feldkamp algorithm for the reconstruction.
If a disk-shaped X-ray volume is reconstructed from an incomplete projection data set, then artifacts occur in the reconstructed X-ray volume, which strongly interfere with a diagnosis of the conditions in the ROI. Therefore, it is desirable to minimize the artifacts in the ROI by recording a complete projection data set.
C-arm X-ray apparatuses with a scanning angle range of the central ray of 180° plus fan angle are known and used predominantly in stationary X-ray diagnostic devices. Typical values of the fan angle in common C-arm systems are values between 10° and 20°. In order to obtain a complete set of projection data for the 3D reconstruction, a rotation angle range of the orbital movement of 200° would be needed in the case of a fan angle of 20°. In C-arm apparatuses in which the C-arm is mounted in a holder so it can be moved along its periphery, the C-arm would have to have an arc length that is increased by the angle range of the holder, that is, 200° plus angle range of the holder. In comparison to a semicircular C-arm with a with a 180° arc length and identical radius of the C-arm, a C-arm with an arc length of 200° plus angle range of the holder has a smaller opening width between the ends of the C-arm. In order to increase the opening width in the case of a given arc length, the radius of the C-arm has to be increased, which, for the purpose of achieving a sufficient stability and torsional stiffness, results in an increase of the weight of the C-arm, and a more stable and heavier construction of the C-arm mobile tripod in order to be able to reliably compensate for the increased tilting torques of the enlarged and heavier C-arm. A C-arm X-ray apparatus with an increased radius is bulkier and heavier and consequently more difficult to handle and maneuver than a compact, mobile C-arm X-ray apparatus with a C-arm length of 180° and a smaller radius with the same opening width. In addition, the advantage of the good mobility of a small C-arm X-ray apparatus no longer exists with larger and heavier C-arm X-ray apparatuses.
DE202005021106U1 relates to a C-arm X-ray apparatus for the automatic generation of projection views for a volume reconstruction, by means of which can be set a stored sequence of adjustment positions of the horizontal, vertical and orbital adjustment axes, which can be adjusted in succession by means of an electric motor, wherein, for each setting, an X-ray projection view with a corresponding projection geometry is recorded.
DE10153787B4 relates to a mobile X-ray diagnostic device with a non-isocentric C-arm that can be moved along its periphery, with a C-arm holder that can be adjusted by means of a motor in terms of least two axes, and with a movement control, which performs the setting of the axes controlled by means of a motor as a function of the position of the C-arm in the orbital axis in such a manner that, with the X-ray recording system, X-ray projection views can be recorded with a predetermined projection geometry. In particular, it is possible to reproduce an isocentric C-arm with a virtual isocenter.
U.S. Pat. No. 4,138,721A relates to a method for generating a limited 3D data set with a focus trajectory for a fan beam, in which the X-ray focus is moved so that the fan beam is moved transversely to the central ray over the ROI and, at the end points of the focus trajectory, the fan beam is located entirely outside of the ROI. The focus trajectory can consist of a line or of a circular arc, the radius of which is considerably smaller than the distance between the X-ray detector and the center of the ROI. Here, the ROI is moved into the radiation field and again out of the radiation field during the recording of X-ray projections.
DE102009031165A1 relates to a method for recording X-ray images of an ROI from several viewing angles for a 3D reconstruction using an X-ray image recording system, in which the X-ray source and the X-ray detector can be positioned separately from one another and aligned relative to one another, wherein the focus of the X-ray source, with recording of X-ray projections, is moved along a focus trajectory, which consists of a combination of straight line segments and/or arc segments, in such a way that the ROI is projected completely onto the X-ray detector at the time of each recording. The line and/or arc segments can be connected to one another and can be located in a plane.
DE10224011A1 discloses a computer-assisted reconstruction method for a three-dimensional object, in which the projection data were generated from an incomplete scan having a scanning range of less than 180°. For the reconstruction, assumptions are made regarding the X-ray transparency of the examination object.
DE102009038787A1 discloses a method for recording a 3D data set of an examination object in order to prevent cutting-off effects, wherein a first scan with a scan angle of 180° plus half a fan angle is recorded, and in the case of a second scan with the same scanning range, the X-ray detector is moved in the scanning direction.
DE4016245C2 relates to a method for recording a complete projection data set using a translation-rotation scanner for an object that exceeds the size of the beam fan.
DE102006037564B3 relates to a method for recording a 3D projection data set, in which, in order to prevent truncation effects, a robot-guided C-arm is tracked synchronously with respect to the rotation in the C-plane in such a manner that the region of interest is located, at least at the time of each rotation angle at which an image recording occurs, within the ray cone of an X-ray bundle of the image recording system.
DE2604020C3 relates to a rotatory scanning of an object with a fan beam and a scanning angle range of 180° plus a fan angle for recording a complete 3D projection data set, in which a diaphragm that can be adjusted depending on the scanning angle position, at the beginning of the scan, first removes a first marginal area of the fan beam and at the end of the scan the second marginal area of the fan beam, which results in a reduction of the patient dose.
DE102011086754A1 relates to a C-arm X-ray apparatus and to a method for the rotatory scanning of an object, in which the rotation of the C-arm is superposed by a shifting movement between the object and the C-arm. In the case of a complete scan with a scanning angle range of 180° plus fan angle, a larger volume can be reconstructed than in the case of a purely rotatory scanning.
U.S. Pat. No. 5,032,990A and the article by K. C. Tam, “Reducing the Fan-Beam Scanning Angular Range,” Phys. Med. Biol., Volume 33 (1988), pp. 955-967, disclose that a mathematically error-free 3D reconstruction can be achieved with 2D projection data that were recorded in the case of a half scan with a flat circular trajectory with a central ray-related rotation angle range 180° plus fan angle.
U.S. Pat. No. 8,284,892B2 relates to a method and to a device for volume reconstruction from projection data that were recorded with a short scan. Redundant projection data are taken into consideration by weighting reconstructed partial volumes before the addition to a total volume.
From the article F. Dennerlein, H. Kunze, J. Boese “Cone-beam reconstruction from a variable-radius planar source trajectory” in 2009 IEEE Nuclear Science Symposium Conference Record (2009), pp. 2496-2499, a reconstruction method of the Feldkamp type is known, in which projection data from flat focus and detector trajectories of a short scan with an angle range of 180° plus fan angle are used. The short scan trajectories can be open rectangles or open trajectories having different radii. It is provided to allow the focus and detector trajectories to oscillate around a rectangular trajectory.
From the article F. Noo, M. Defrise, R. Clackdoyle, H. Kudo “Image reconstruction from fan-beam projections on less than a short scan,” Phys. Med. Biol. 47 (2002) 2525-2546, published in July 2002, a super short scan method for recording a projection data set and for reconstructing a decentrally arranged ROI is known, wherein the scan angle around the rotation center can be less than 180° plus fan angle.
The methods for recording a scan for generating a projection data set with a C-arm X-ray apparatus with cone beam geometry that are known in the prior art, and wherein the apparatus in the plane of the C-arm has a fan beam geometry with a fan angle and the C-arm of which during the scan is moved in a space-fixed plane, have the disadvantage that a complete set of X-ray projections for the analytical reconstruction of a disk-shaped X-ray volume located in the plane of the C-arm requires an orbital angle adjustment range of at least 180°.
The aim of the invention is to provide a method for recording a scan for generating an X-ray projection view with a C-arm X-ray apparatus, wherein the C-arm X-ray apparatus has a cone beam geometry and in the plane of the C-arm fan beam geometry with a fan angle and an orbital angle adjustment range, and the C-arm of which, during the scan, is moved in a space-fixed plane, so that with the X-ray projection views, a complete set of X-ray projections is produced, for the analytical reconstruction of a disk-shaped X-ray volume of the central layer, which is located in the plane of the C-arm, with an orbital angle adjustment range of less than 180°.